1. Field of the Invention
The following description relates to a holographic storage medium with an increased recording capacity, recording and/or reproducing apparatuses used with the holographic storage medium, and recording and/or reproducing methods used with the holographic storage medium.
2. Description of the Related Art
In optical holography, data is not stored on a surface of a recording medium, but is stored in a volume thereof. A signal beam interferes with a reference beam within the recording medium to generate a plurality of interference gratings referred to as a data page. The interference gratings change the optical characteristics of the reference beam, causing overlapping to occur. This process is referred to as multiplexing. When data is read from the recording medium, a single reference beam is incident on the recording medium under the same conditions as the conditions used during the data recording to generate a diffraction beam having the stored data page. The diffraction beam is detected by a detection array, which extracts a stored plurality of data bits from a measured intensity pattern. The data page contains the data bits, which are also referred to as pixels. As such, when the data pages overlap in the volume of the recording medium, data storage capacity is increased.
Referring to FIG. 1A, data is recorded to a holographic storage medium 100 using a signal beam S to carry the data and a reference beam R. During recording of the hologram 100, as illustrated in FIG. 1A, the reference beam R and the signal beam S interfere with each other to generate an interference pattern, which is transferred to the holographic storage medium 100. During reproduction of data from the holographic storage medium 100, as illustrated in FIG. 1B, the original reference beam R is radiated onto the recorded as a hologram on the holographic storage medium 100, and the recorded hologram recorded on the holographic storage medium 100 diffracts the reference beam to output the signal beam S. However, when the reference beam R used to reproduce data is different from an original beam used during the recording of data, the intensity or direction of a reproduced beam is different from the intensity or direction of the original recorded beam. Generally, as such a difference increases, the intensity of radiation is reduced by a shape defined by a sinc function.
FIG. 2 is a view illustrating angles according to regions of a signal beam S when data is recorded on the holographic storage medium 100. Referring to FIG. 2, the signal beam S and a reference beam R are incident on the holographic storage medium 100. The signal beam S is modulated by an optical modulator, such as a spatial light modulator (SLM) 210, and focused in a shape of a page on the holographic storage medium 100. The SLM 210 has a surface with a flat shape. Angles of the signal beam S incident on the holographic storage medium 100 are different according to regions of the SLM 210. The SLM 210 is classified into a, b, a and d regions illustrated in FIG. 2 along a scanning direction of the reference beam R. An incidence angle and selectivity (i.e., a Bragg selectivity angle) of the signal beam S of each region may be calculated. The results are shown in Table 1.
TABLE 1region aregion bregion cregion dincidence angle35.8628.6221.3814.14of signal beamS (°)selectivity (°)0.110.120.140.16
The incidence angle, at which the signal beam S is incident on the holographic storage medium 100, is an angle of the signal beam S with respect to a normal direction of the holographic storage medium 100 (i.e., a direction perpendicular to a recording surface of the holographic storage medium 100). The incidence angle is 35.86° in a region “a” of the SLM, 28.62° in a region “b,” 21.38° in a region “c,” and 14.14° in a region “d.” Thus, there is an angle difference of about 7.24° between adjacent regions. This angle difference is used because the signal beam S passes through an objective lens (not shown) having a numerical aperture (NA) before the signal beam S is incident on the holographic storage medium 100.
When a beam is incident at an angle greater than a predetermined angle with respect to the central axis, the beam is refracted outwards. The NA of the objective lens is proportional to a sine of the predetermined angle. When the beam is incident on the objective lens at an angle less than or equal to the predetermined angle, the beam is not refracted outwards, and instead is totally reflected so as to be spread within the objective lens. When the selectivity of the signal beam S of each region is calculated, the smaller the incidence angle is, the greater the selectivity is. That is, the selectivity of the signal beam S varies according to the region of the SLM. Generally, high selectivity is desirable in order to reduce crosstalk. However, as the selectivity is increased, the incidence angle at which data can be recorded is decreased, preventing high density recording and multiplexing of data to the holographic storage medium 100.
FIG. 3 is a view illustrating the case where a page 300 is divided into regions. Referring to FIG. 3, a signal beam S is modulated by the SLM 210 to have a shape of the page 300 and is divided into A, B, C and D regions along a scanning direction of a reference beam R. FIG. 4 is a graph illustrating the selectivity measured in the regions A, B, C, and D illustrated in FIG. 3. Referring to FIG. 4, the selectivity varies according to the region, similar to the results shown in Table 1. Thus, crosstalk values are different according to the region of the page 300. Maximum selectivity is selected and a recording interval is determined based on the selected maximum selectivity in order to minimize crosstalk between pages. As a result, realization of high density data recording is difficult.